Platinum (II) complexes, preparation and use

ABSTRACT

The invention relates to a method for the preparation of platinum (II) complexes, in particular dicarboxylatoplatinum (II) complexes containing a neutral bidentate ligand, such as oxaliplatin. The method includes the step of reacting a bis-dicarboxylatoplatinate (II) species with a suitable neutral bidentate ligand to form a neutral dicarboxylatoplatinum (II) complex and, if necessary, recrystallising the product to form a pure dicarboxylatoplatinum (II) complex containing a neutral bidentate ligand. The invention also relates to a method for producing a bis-dicarboxylatoplatinate (II) species, and to new platinum (II) complexes that can be made by the method of the invention.

BACKGROUND OF THE INVENTION

THIS invention relates to the preparation of platinum(II) complexes, in particular the preparation of dicarboxylatoplatinum(II) complexes containing a neutral bidentate ligand, (such as oxaliplatin, which has become increasingly important due to its anti-cancer activity).

Dicarboxylatoplatinum(II) complexes (such as oxaliplatin) containing a neutral bidentate ligand (“non-leaving group”) have in the past been synthesized by way of a process that utilizes a silver salt to remove halide ions from the complex. The use of a silver compound in the process results in numerous contaminants, which must be removed by further processes in order to achieve purity that is suitable for anti-cancer pharmaceutical agent purposes.

Oxaliplatin and its pharmaceutical properties were first disclosed by Kidani et al. in J Med Chem, 1978, 21, 13135 and in U.S. Pat. No. 4,169,846. In this patent a halogenoplatinum compound is used as the starting material. Halide ions are removed by a silver salt, whereafter an oxalate is introduced either as the free acid or a salt thereof.

In general, a method for the production of oxaliplatin is as set out below:

Step 1. 1 mol K₂PtX₄+1 mol L→PtLX₂ X=Cl, Br, I and L=trans-l-1,2-diaminocyclohexane

Step 2. 1 mol PtLX₂+2 mol AgNO₃→PtL(H₂O)₂ ²⁺+2AgX+2NO₃ ⁻ or 1 mol PtLX₂+1 mol Ag₂Y→PtL(H₂O)₂ ²⁺+2AgX+Y²⁻ Y=SO₄ ²⁻

Step 3. PtL(H₂O)₂ ²⁺+Z₂(oxalate)→PtL(oxalate)+2Z⁺ Z=K⁺, Na⁺ or H⁺

U.S. Pat. No. 5,290,961 in the name of Tanaka Kikinzoku Kogyo K.K. teaches that the abovementioned method has the disadvantage that many impurities are incorporated into the products. These impurities include unreacted PtLX₂, AgX and Ag⁺. The presence of PtLX₂ is attributed to their generally insoluble nature in water. As a result, large quantities of water must be used in step 2 to dissolve PtLX₂. This prevents the AgX, even though it is insoluble in water, from being completely removed from the solution. U.S. Pat. Nos. 5,338,874 and 5,420,319, also in the name of Tanaka Kikinzoku Kogyo K.K., teach processes for the production of cis-oxalato(trans-l-1,2-cyclohexanediamine)platinum(II) with high optical purity which can be used as an active pharmaceutical ingredient of a carcinostatic agent. However, these processes also follow complicated multi-step pathways, making use of silver compounds which must also ultimately be removed from the process.

SUMMARY OF THE INVENTION

The present invention has been made to address the drawbacks of the prior art, in terms of which dicarboxylatoplatinum(II) complexes containing a neutral bidentate ligand, such as oxaliplatin, are produced using the silver method, which results in contaminants in the final product, is time consuming and expensive. Furthermore, the absence of silver compounds in the process enables the synthesis of a group of novel platinum compounds i.e ligands containing donor atoms other than N such as S or Se.

A first aspect of the invention relates to a method for the preparation of a platinum(II) complex, in particular a dicarboxylatoplatinum(II) complex containing a neutral bidentate ligand, such as oxaliplatin, the method including the step of reacting a bis-dicarboxylatoplatinate(II) species with a suitable neutral bidentate ligand to form a neutral dicarboxylatoplatinum(II) complex and if necessary recrystallising the product to form a pure dicarboxylatoplatinum(II) complex containing a neutral bidentate ligand.

The bis-oxalatoplatinate(II) species and ligand are typically reacted at a temperature of 40° C. to 100° C., preferably approximately 95° C., for a period of 0.5 to 3 hours, preferably 1 hour.

Any dicarboxylatoplatinate(II) species may be removed from the product by dissolving the product in distilled water and adding an oxalate such as Cs₂C₂O₄, which transforms the dicarboxylatoplatinate(II) species into a species that can be separated from the dissolved product by filtration.

The neutral bidentate ligand is typically an amine.

The amine may be a diamine.

Where the method is for the preparation of chemically and optically pure oxaliplatin, the ligand is optically pure trans-l-1,2-diaminocyclohexane.

The neutral bidentate ligand may contain donor atoms other than N, or N together with a donor atom other than N, typically S and Se, for example:

-   -   neutral bidentate heterocyclic amines with an S donor atom (for         example thioethereal groups), such as:

1-alkyl/aryl-2-alkylthioalkyl/aryl heterocyclic amines, particularly imidazoles or pyridines, for example:

Ligand (i) 1-methyl-2-methylthioethylimidazole Ligand (ii) 1-methyl-2-methylthiopropylimidazole Ligand (iii) 1-butyl-2-methylthiomethylimidazole Ligand (iv) 1-methyl-2-methylthiomethylimidazole Ligand (v) 1-butyl-2-methylthioethylimidazole Ligand (vi) 2-methylthiomethylpyridine Ligand (vii) 2-methylthioethylpyridine Ligand (viii) 2-methylthiopropylpyridine;

-   -   aminoalkylthioalkyl/aryl compounds for example:

Ligand (ix) 1-amino-2-thiomethylethane Ligand (x) 1-amino-2-thioethylethane;

-   -   dithioethers for example:

-   Ligand (xi) 2,5-dithiahexane;     -   diseleno ethers for example:

-   Ligand (xii) 2,5-diseleno hexane; etc.

New oxalatoplatinum(II) complexes containing S or Se donor atoms that can be prepared using the method of the invention include:

Complex oxalato(1-methyl-2-methylthioethylimidazole)platinum(II) (i) Complex oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II) (ii) Complex oxalato(1-butyl-2-methylthiomethylimidazole)platinum(II) (iii) Complex oxalato(1-methyl-2-methylthiomethylimidazole)platinum(II) (iv) Complex oxalato(1-butyl-2-methylthioethylimidazole)platinum(II) (v) Complex oxalato(2-methylthiomethylpyridine)platinum(II) (vi) Complex oxalato(1-amino-2-thioethylpyridine)platinum(II) (vii) Complex oxalato(1-amino-2-thiopropylpyridine)platinum(II) (viii) Complex oxalato(1-amino-2-thiomethylethane)platinum(II) (ix) Complex oxalato(1-amino-2-thioethylethane)platinum(II) (x) Complex oxalato(2,5-dithiahexane)platinum(II) (xi) Complex oxalato(2,5-diseleno hexane)platinum(II). (xii)

The above new complexes may be used in methods of treating cancer in patients, and in methods of manufacturing medicaments for treating cancer in patients

The complexes produced according to the method of the invention contain no traces of silver.

A second aspect of this invention is a method for producing a bis-dicarboxylatoplatinate(II) species, e.g. a bis-oxalatoplatinate(II) salt which may be used in the method of the first aspect of the invention. The method according to the second aspect of the invention includes the step of either reacting a platinum(II) compound, such as K₂PtX₄ or reacting a platinum(IV) compound such as K₂PtX₆ where X is a halide such Cl, Br or I, preferably Cl, with a dicarboxylate such as an oxalate, wherein the platinum(II) or platinum(IV) compound and oxalate salt are reacted at a high mole ratio of greater than 1:4, preferably 1:8 or greater, more preferably 1:16 or greater most preferably 1:24 or greater.

In the case of the platinum(IV) compound, this compound is reduced to platinum(II) by the oxalate, or it may be reduced by another reducing agent such as SO₂ or sulfite.

The oxalate is typically K₂C₂O₄.

The platinum(II) bis-dicarboxylato species is typically K₂Pt(C₂O₄)₂.2H₂O.

The platinum(II) compound or platinum(IV) compound and oxalate are typically reacted at a temperature of from 40° C. to less than 100° C., preferably approximately 95° C., for a period of 0.5 to 4 hours, typically 1 hour.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph indicating the efficiency of the synthesis of K₂Pt(C₂O₄)₂.2H₂O relative to the ratio of K₂C₂O₄ to K₂PtCl₆ in a reaction of K₂PtCl₆ with K₂C₂O₄ with a constant reaction time of 1 h 15 min at 95° C.;

FIG. 2 is a graph indicating the time taken to reach an 85% yield of K₂Pt(C₂O₄)₂.2H₂O relative to the oxalate to platinum ratio in the reaction of K₂C₂O₄ with K₂PtCl₆ at 95° C.;

FIG. 3 is a chromatographic analysis of the oxaliplatin product which did not dissolve when suspended in 6 ml water, in Example 4;

FIG. 4 is a chromatographic analysis of the oxaliplatin product of FIG. 3 which has subsequently been washed with water, in Example 4; and

FIG. 5 is a chromatographic analysis of the chemically pure oxaliplatin product of Example 4 as determined on a chiral column indicating its optical purity.

DESCRIPTION OF PREFERRED EMBODIMENTS

This invention relates to a method for the preparation of platinum(II) complexes, in particular a dicarboxylatoplatinum(II) complex containing a neutral bidentate ligand (“non-leaving group”) such as oxaliplatin, and new platinum(II) complexes that include a donor atom other than N, the method includes the steps of:

-   -   Step 1. the production of a bis-dicarboxylatoplatinate(II)         species, typically K₂Pt(C₂O₄)₂.2H₂O;     -   Step 2. reaction of the bis-dicarboxylatoplatinate(II) species         with a suitable neutral bidentate ligand to form a         dicarboxalatoplatinum(II) complex containing a neutral bidentate         ligand product; and if necessary     -   Step 3. recrystallising the product to form a pure         oxalatoplatinum(II) complex containing a neutral bidentate         ligand product.

Step 1

The reaction times quoted in all of these steps are given for reagent quantities of ˜10 g.

A bis-dicarboxylatoplatinate(II) species such as K₂Pt(C₂O₄)₂.2H₂O may be synthesized using either K₂PtCl₆, or K₂PtCl₄ as starting materials. It may be possible to use H₂PtCl₆ as a starting material.

Method 1—synthesis of K₂Pt(C₂O₄)₂.2H₂O using K₂PtCl₄

K₂PtCl₄ and K₂C₂O₄ may be dissolved in distilled water in the mole ratio of 1:16 and stirred at a temperature of between 40° C. to 100° C., typically approximately 95° C. for 0.5 to 2 hours, typically 1 hour. The solution is refrigerated at a temperature from 2° C. to 10° C., typically approximately 5° C. for 0.5 to 2 hours typically 1 hour to complete crystallization where after it may be filtered. The precipitated product may be washed 5 times with small volumes of cold water and once with acetone and allowed to air dry. 1 mol K₂PtCl₄+16 mol K₂C₂O₄→K₂Pt(C₂O₄)₂.2H₂O

Method 2—synthesis of K₂Pt(C₂O₄)₂.2H₂O using K₂PtCl₆ (oxalate acting as reducing and complexing agent)

K₂C₂O₄ and K₂PtCl₆ may be suspended in water in a mole ratio of 34:1 and stirred at a temperature of between 40° C. to 100° C., typically approximately 95° C. for 0.5 to 4 hours typically 55 minutes as illustrated by the curve in FIG. 2. Only 10 minutes additional time is required for a ratio of 24:1. The reaction vessel may be maintained at a temperature between 2° C. to 10° C., typically approximately 5° C. for 0.5 to 2 hours typically 1 hour to allow for complete crystallization followed by filtration. The precipitate may be washed 5 times with small volumes of cold water and once with acetone and allowed to air dry. 1 mol K₂PtCl₆+34 mol K₂C₂O₄→K₂Pt(C₂O₄)₂.2H₂O

In this reaction the oxalate plays two roles. Firstly, it acts as a reducing agent. On heating the solution of K₂C₂O₄ with a suspension of K₂PtCl₆ the latter dissolves to form a dark orange solution with the evolution of gas. The disappearance of the starting material accompanied with a darkening of the solution indicates the formation of PtCl₄ ²⁻ therefore a reduction of platinum(IV) to platinum(II). The evolution of gas further indicates the oxidation of oxalate to CO₂.

After all the K₂PtCl₆ has dissolved, the oxalate starts a second role where it acts as a complexing agent and a light yellow precipitate starts to form. Thus the oxalate acts as a suitable complexing agent which can coordinate to the platinum to form K₂Pt(C₂O₄)₂.2H₂O. Therefore, this method can be divided into two separate reactions. An alternative reducing agent such as SO₂ may be used in the place of the oxalate, to reduce the platinum IV to platinum II.

In prior art methods which describe the synthesis of K₂Pt(C₂O₄)₂.2H₂O, a small excess of K₂C₂O₄ of up to 4 times was used at a temperature of 100° C. for an extensive period (up to 18 hrs). See Shriver DF (Ed) 1979. Inorganic Synthesis, Vol. XIX: 16-17. During that time reduction of the platinum species occurs forming platinum metal (platinum black).

In accordance with an aspect of the method of this invention, the inventor has quite unexpectedly found out that when a platinum compound and oxalate are reacted at a high mole ratio of greater than 1:4, preferably greater than 1:8, more preferably greater than 1:16, most preferably 1:24 or greater and lower reaction temperatures (less than 100° C., typically 95° C.), shorter reaction times are attained and no reduction to platinum metal (no platinum black) is observed. The higher concentration of the complexing anion, oxalate, not only acts as a stabilizater of the bis-oxalatoplatinate(II) species but also improves reaction rates as well as ligand exchange thus resulting in high yields of the bis-oxalatoplatinate(II) species. The larger the excess oxalate used, the higher the percentage yield of K₂Pt(C₂O₄)₂.2H₂O. (See FIG. 1), when a 1:16 ratio of K₂C₂O₄ is used relative to K₂PtCl₆, the yield of K₂Pt (C₂O₄)₂.2H₂O is only 67%. The yield consistently increases as the oxalate excess increases such that a ratio of 34:1 results in a 86% yield (See FIG. 2). In FIG. 2 the ratio of platinum to potassium oxalate is plotted against the time in minutes required to reach the maximum yield of the production of K₂Pt(C₂O₄)₂.2H₂O from K₂PtCl₆, namely about 85%. Experiments performed with ratios of 8:1 or lower resulted in the formation of finely divided black platinum metal due to decomposition. This gradually occurs upon heating for approximately 8½ hours. When ratios of 3:1-8:1 are employed, extensive reaction time periods are required to reach maximum efficiencies of conversions, which still results in low yields. In the literature, a ratio of 3:1 results in a 30% yield after refluxing for 18 hours (1080 mins) (Synthesis of K₂Pt(C₂O₄)₂.2H₂O, Shriver, D. F., Inorganic Synthesis, 19, pp. 16-17, (ed.). 1979.). As the amount of oxalate increases, the time taken to reach the 85% yield decreases from 156 mins at a mole ratio 12:1, to 110 mins at a mole ratio of 16:1, to 65 mins at a mole ration of 24:1 to 55 mins at a mole ratio of 34:1.

Step 2

The bis-dicarboxylatoplatinate(II) species viz. K₂Pt(C₂O₄)₂.2H₂O, may be reacted with a suitable, preferably optically pure, neutral bidentate ligand to form a neutral platinum(II) oxalato compound such as oxaliplatin.

K₂Pt(C₂O₄)₂.2H₂O may be dissolved in a suitable solvent system whereafter a ligand (L) dissolved in a suitable solvent is added. The ligand may be selected from any bidentate neutral donor ligand, but in the case of oxaliplatin is a diamine, namely optically pure trans-l-1,2-diaminocyclohexane. Optically pure trans-l-1,2-diaminocyclohexane may be obtained in an optically pure state by crystallization with tartaric acid, for example by a method described by Hans-Jörg Schanz, Michael A. Linseis and Declan G. Gilheany, in Improved resolution methods for (R,R)- and (S,S)-cyclohexane-1,2-diamine and (R)- and (S)-Binol. Tetrahedron Asymmetry 12 (2003), 2763-2769, the content of which is incorporated herein by reference. The solution may be maintained at a temperature from 40 to 100° C., typically approximately 95° C. for 0.5 to 2 hours, typically 1 hour to form a precipitate which contains oxalatoplatinum(II) complex such as oxaliplatin. 1 mol K₂Pt(C₂O₄)₂.2H₂O+1 mol L→PtL(C₂O₄)+K₂C₂O₄

Step 3

The crude product may be purified by extracting the oxalatoplatinum(II) complex with sufficient excess of water. Contaminating K₂Pt(C₂O₄)₂-2H₂O which has similar solubility properties to the oxalatoplatinum(II) complex such as oxaliplatin at low temperatures may be removed by transforming it into Cs₂Pt(C₂O₄)₂ through addition of Cs₂C₂O₄ which upon cooling removes Pt(C₂O₄)₂ ²⁻. The filtrate of this solution upon vacuum evaporation leaves a solid which can be washed with a small portion of hot water removing the residual amounts of Pt(C₂O₄)₂ ²⁻and oxalate salts; The white solid may be washed with cold water to obtain pure oxaliplatin. A further amount of oxalatoplatinum(II) complex may be obtained from the filtrate after removing Cs₂Pt(C₂O₄)₂ which precipitates after cooling. The final step consists of the recrystallization of the above white precipitate. A final oxaliplatin product has a chemical purity of >99.5% and optical purity of >99.98%. The overall yield of chemically and optically pure oxaliplatin is 15%.

Thus, the above method of the invention when used for producing oxaliplatin uses only 5 steps with an overall reaction time of 16 hours. It also requires the use of only four different chemicals, namely: K₂PtCl₆/K₂PtCl₄, K₂C₂O₄, Cs₂C₂O₄ and a suitable neutral bidentate ligand.

The method described above may be used to form many other platinum(II) complexes with neutral bidentate ligands, and makes it possible to form platinum(II) complexes with neutral bidentate ligands that contain donor atoms other than N, typically S and Se, for example:

-   -   neutral bidentate heterocyclic amines with an S donor atom, such         as thioethereal S containing compounds of the general formula:

1-alkyl/aryl-2-alkylthioalkyl/aryl heterocyclic amines, particularly imidazoles or pyridines;

-   -   aminoalkylthioalkyl/aryl compounds;     -   dithioethers for example 2,5-dithiahexane;     -   diseleno ethers for example 2,5-diseleno hexane; etc.

Ligands containing S and Se donor atoms cannot be used in reactions that make use of silver compounds, because these atoms react by binding very strongly with both platinum and silver ions.

The following 2-methylthioalkyl imidazole and pyridine neutral bidentate ligands:

Ligand (i) 1-methyl-2-methylthioethylimidazole Ligand (ii) 1-methyl-2-methylthiopropylimidazole Ligand (iii) 1-butyl-2-methylthiomethylimidazole Ligand (iv) 1-methyl-2-methylthiomethylimidazole Ligand (v) 1-butyl-2-methylthioethylimidazole Ligand (vi) 2-methylthiomethylpyridine Ligand (vii) 2-methylthioethylpyridine Ligand (viii) 2-methylthiopropylpyridine (prepared by the methods described in J G H du Preez, T I A Gerber, W Edge, V L V Mtotywa and B J A M van Brecht. Nitrogen Reagents in Metal Ion Separation. XI. The Synthesis and Extraction Behaviour of a New NS imidazole Derivative. Solv. Extr. & Ion Exch. (2001) 19(1), 143-154) (the content of which is incorporated herein by reference) may be used in the below method to prepare the 2-methylthioakyl complexes of imidazole and pyridine (i) to (v) mentioned below.

K₂Pt(C₂O₄)₂.2H₂O may be dissolved in distilled water over a 90° C. water bath to which is added dropwise while stirring one molar equivalent of the relevant neutral bidentate ligand dissolved in acetone. The platinum(II) solution so formed may be stirred for 1½ hours at 90° C. and subsequently allowed to cool. The resultant precipitate may be filtered and washed once with cold distilled water and air dried in an oven at 50° C.

Examples of 2-methylthioalkyl complexes of imidazole prepared by the above method are reflected in the structural Formula (I) below where R₁ and R₂ may be selected from alkyl (e.g. CH₃, C₂H₅ etc.) and aryl (e.g. phenyl) groups. Typical 2-methylthioakyl complexes of imidazole are complexes (i) to (v) below:

The chemical names for the complexes (i) to (v) are:

Complex oxalato(1-methyl-2-methylthioethylimidazole)platinum(II) (i) Complex oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II) (ii) Complex oxalato(1-butyl-2-methylthiomethylimidazole)platinum(II) (iii) Complex oxalato(1-methyl-2-methylthiomethylimidazole)platinum(II) (iv) Complex oxalato(1-butyl-2-methylthioethylimidazole)platinum(II). (v)

Examples of 2-methylthioalkyl complexes of pyridine of the invention are reflected in the structural Formula (II) below where R₂ may be selected from alkyl (e.g. CH₃, C₂H₅ etc.) and aryl (e.g. phenyl) groups. Typical 2-methylthioalkyl complexes of pyridine are compounds (vi) to (viii) below:

The chemical names for the complexes (vi) to (viii) are:

Complex (vi) oxalato(2-methylthiomethylpyridine)platinum(II) Complex (vii) oxalato(2-methylthioethylpyridine)platinum(II) Complex (viii) oxalato(2-methylthiopropylpyridine)platinum(II).

2-methylthioakyl complexes of imidazole and pyridine mentioned above have been shown to have anti-cancer properties.

The following ligands:

Ligand (ix) 1-amino-2-thiomethylethane Ligand (x) 1-amino-2-thioethylethane may be used to prepare an aliphatic aminothioether complex of Pt(II)oxalate, using the method below:

K₂Pt(C₂O₄)₂.2H₂O may be dissolved in distilled water over a 90° C. water bath. Dimethylformamide (dmf) may be added to the platinum(II) solution to form a 20:80 water:dmf mixed solvent ratio. One molar equivalent of the relevant ligand may be dissolved in acetone and added dropwise while stirring whereafter the platinum(II)solution so formed may be stirred for 2 hours at 90° C. The reaction may be allowed to cool at room temperature and the resultant precipitate filtered, washed once with cold distilled water and allowed to air dry in an oven at 50° C. to produce a light yellow product (47% yield).

Examples of aliphatic aminothioether complexes of Pt(II)oxalate prepared by the above method are:

Complex (ix) oxalato(1-amino-2-thiomethylethane)platinum(II) Complex (x) oxalato(1-amino-2-thioethylethane)platinum(II).

The conventional method of preparing oxaliplatin, such as the method described in U.S. Pat. No. 5,420,319, uses 6 steps and has an overall reaction time of 38 hours. The method of the present invention reduces the reaction time and has fewer steps. Thus, the method of the invention is simpler, more efficient and more cost-effective than conventional methods. The method of the invention also eliminates the contamination problems experienced in conventional methods, such as that described in U.S. Pat. No. 5,420,319. No PtLX₂ is formed which is generally quite insoluble in water and no silver is used which needs to be removed. The only by-products that need to be eliminated from the method of the invention is excess K₂C₂O₄ when synthesizing K₂Pt(C₂O₄)₂.2H₂O and unreacted K₂Pt(C₂O₄)₂.2H₂O and oxalate salts when producing PtL(C₂O₄) in the case of oxaliplatin. The excess oxalate salts are very soluble in water and can be removed easily by washing with distilled water.

A further advantage of not using a silver compound is that this reaction need not be carried out in darkness and can be used with neutral bidentate ligands that contain donor atoms other than N, such as S and Se which readily react with silver ions. Such ligands cannot be used in the methods that make use of silver compounds, thus preventing the synthesis of their carboxylato analogues. Thus, the method of the invention makes possible, inter alia, the synthesis of bidentate N,N ligand oxalato complexes of platinum(II) and a variety of new oxalatoplatinum(II) complexes containing bidentate ligands with N, S or Se donor atoms. These compounds, according to test results, have application as new anticancer agents which may be used in methods of treating cancer in patients, and in methods of manufacturing medicaments for treating cancer in patients.

The invention will now by described with reference to the following non-limiting examples.

EXAMPLE 1 Production of K₂Pt(C₂O₄)₂.2H₂O from K₂PtCl₄

2.075 g (4.6 mmole) K₂PtCl₄ and 13.561 g (73.6 mmole) K₂C₂O₄ was dissolved in 30 ml distilled water and stirred for 1 hour at approximately 95° C. The reaction vessel was subsequently refrigerated at approximately 5° C. for 2 hours and the precipitate filtered, washed 5 times with 4 ml water, rinsed with 2 ml acetone and air dried in a 50° C. oven. The yield was 98%.

EXAMPLE 2 Production of K₂Pt(C₂O₄)₂.2H₂O from K₂PtCl₆ with Oxalate as the Reducing Agent

1.425 g (2.9 mmole) K₂PtCl₆ and 18.392 g (99.8 mmole) K₂C₂O₄ was dissolved in 50 ml distilled water at approximately 95° C. and stirred for 1 hour 15 minutes, subsequently cooled in a refrigerator for 2 hours and filtered. The excess K₂C₂O₄ was removed by washing the precipitate 5 times with 3 ml portions of water and rinsed once with 2 ml acetone. It was allowed to air dry in a 50° C. oven. The yield was 86%.

EXAMPLE 3 Production of K₂Pt(C₂O₄)₂.2H₂O from K₂PtCl₆ with SO₂ as the Reducing Agent

0.326 g (0.67 mmole) K₂PtCl₆ was suspended in 6 ml distilled water at approximately 95° C. A saturated SO₂ solution was added drop wise till all the K₂PtCl₆ dissolved. Excess K₂C₂O₄ (1.981 g (10.8 mmole) was added directly to the platinum solution and stirred for a further hour. The mixture was refrigerated for 2 hours and filtered. The precipitate was washed 5 times with 3 ml portions of water, rinsed once with 1 ml acetone and allowed to air dry in a 50° C. oven. The yield was 78%.

EXAMPLE 4 Synthesis of Oxaliplatin

K₂Pt(C₂O₄)₂.2H₂O (4.50 g, 9.97 mmol) was dissolved in distilled water (815 ml) at 90° C. to provide a platinum oxalate solution. One equivalent of optically pure trans-l-1,2-diaminocyclohexane (produced by a method as described in Hans-Jörg Schanz, Michael A. Linseis and Declan G. Gilheany, Improved resolution methods for (R,R)- and (S,S)-cyclohexane-1,2-diamine and (R)- and (S)-Binol. Tetrahedron Asymmetry 12 (2003), 2763-2769) (1.06 g, 9.27 mmol) was dissolved in distilled water (74 ml) and the pH adjusted with dilute acetic acid to between 6.6-7.5, to provide an amine solution. The amine solution was added incrementally in eight equal portions every 45 minutes to the platinum oxalate solution over 6 hours where after the solution was cooled to room temperature and the precipitate filtered. The solvent was removed from the filtrate under vacuum. The solid obtained from the filtrate contains oxaliplatin in a crude form.

This solid was suspended in distilled water (40 ml) over a water bath at 70° C. for a period of time (10 minutes) where after the remaining solid was removed by filtration. The filtrate was cooled and 0.75 molar equivalents of Cs₂C₂O₄ in relation to K₂Pt(C₂O₄)₂.2H₂O was added. The solution was stirred for approximately 20 minutes and the precipitate which formed was filtered. The solvent was subsequently vacuum evaporated. The remaining solid was suspended in a small portion of distilled water (6 ml) and heated to 70° C. for a period of time (10 minutes) and filtered to provide a filtrate (A). The solid which did not dissolve contained mostly oxaliplatin with a small portion of oxalate salt (see FIG. 3). This was readily purified by washing the solid with a small portion (3 ml) of distilled water to obtain pure oxaliplatin (see FIG. 4).

The filtrate (A) was cooled to 5° C. and the precipitate filtered. The solvent was vacuum evaporated. The solid which remained contained mainly oxaliplatin and cesium and potassium oxalates. The oxalate salts were readily removed by washing 5 times with small portions of distilled water (3 ml) resulting in a further amount of pure oxaliplatin.

All the oxaliplatin samples produced had a chemical purity >99.5% (see FIG. 4) and optical purity of >99.98% (see FIG. 5).

The overall yield of chemically and optically pure oxaliplatin was 15%.

EXAMPLE 5 Method for Producing Oxaliplatin with K₂PtCl₄ as the Starting Material

Both K₂PtCl₄ (41.5 g; 92.0 mmole) and 271.17 g (1178.8 mmole) of K₂C₂O₄ were simultaneously dissolved in 600 ml water and kept at 95° C. for at least 1 hour, during which time K₂Pt(C₂O₄)₂.2H₂O gradually precipitated. The solution was cooled for over 2 hours at 5° C., filtered and washed with 5 80 ml portions of water and finally with acetone. After 45 minutes in an oven at 50° C. pure dry K₂Pt(C₂O₄)₂.2H₂O (43.72 g; 89.96 mmole) was obtained. The product was dissolved in 120 ml pure water at 95° C. and trans-l-1,2-diaminocyclohexane (10.28 g; 90.03 mmole) dissolved in 40 ml acetone was added and stirred for 1½ hours at 95° C. It was left to stand at room temperature for over 2 hours, The solvent was removed from the filtrate under vacuum. The solid obtained from the filtrate contained most of the oxaliplatin in a crude form.

This solid was suspended in distilled water (388 ml) over a water bath at 70° C. for a period of time (20 minutes) where after the remaining solid was removed by filtration. The filtrate was cooled and 0.75 molar equivalents of Cs₂C₂O₄ in relation to K₂Pt(C₂O₄)₂.2H₂O was added. The solution was stirred for approximately 20 minutes and the precipitate which formed was filtered. The solvent was subsequently vacuum evaporated. The remaining solid was suspended in a portion of distilled water (58 ml) and heated at 70° C. for a period of time (20 minutes) and filtered to provide a filtrate (A).

The solid which did not dissolve contained mostly oxaliplatin with a portion of oxalate salt. This was readily purified by washing the solid with a portion (29 ml) of distilled water to obtain pure oxaliplatin.

The filtrate (A) was cooled to 5° C. and the precipitate filtered. The solvent was vacuum evaporated. The solid which remained contained mainly oxaliplatin and cesium and potassium oxalates. The oxalate salts were readily removed by washing 5 times with portions of distilled water (29 ml) resulting in pure oxaliplatin.

All the oxaliplatin samples produced had a chemical purity >99.5% and optical purity of >99.98%. The overall yield of chemically and optically pure oxaliplatin was 15%.

EXAMPLE 6 Method for the Preparation of 2-methylthioakyl Complexes of Imidazole and Pyridine, in this Case oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II).

0.68 g of K₂Pt(C₂O₄)₂2H₂O was dissolved in 40 ml distilled water over a water bath at 90° C. to which was added dropwise while stirring 0.638 g of 1-methyl-2-methylthiopropylimidazole dissolved in 4 ml acetone. The platinum(II) solution was stirred for 1½ h at 90° C. and subsequently allowed to cool overnight. The resultant precipitate was filtered and washed once with a 6 ml portion of cold distilled water and air dried in an oven at 50° C. Light yellow solid (60% yield).

EXAMPLE 7 Method for the Preparation of an Aliphatic Aminothioether Complex of Pt(II) Oxalate, in this Case oxalato(1-amino-2-thiomethylethane)platinum(II).

K₂Pt(C₂O₄)₂.2H₂O (0.6 mmol; 0.291 g) was dissolved in 8 ml distilled water over a 90° C. water bath. 32 ml dmf was added to the platinum(II) solution to form a 20:80 water:dmf solvent ratio. One molar equivalent (0.6 mmol) of the 1-amino-2-thiomethylethane ligand was dissolved in 3 ml acetone and added dropwise while stirring whereafter the platinum(II) solution was stirred for 2 hours at 90° C. The reaction was allowed to cool overnight at room temperature and the resultant precipitate filtered, washed once with a 3 ml portion of cold distilled water and allowed to air dry in an oven at 50° C. Light yellow product 47% yield.

EXAMPLE 8

The purities of the compounds of Examples 6 and 7 were determined by C, H and N analysis, infrared spectroscopy and the ligands by NMR spectroscopy. All the complexes show molecular weight peaks obtained from FAB mass spectral data which correspond with monomolecular complexes except Pt(1-methyl-2-methylthiopropylimidazole)C₂O₄ which gave only fractions.

EXAMPLE 9

Anticancer testing on the complex of Example 6 was performed and compared with cisplatin under similar conditions using 5% foetal calf serum as a medium. The percentage inhibition of oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II) on cervical cancer cells was 88.6% and 97.7% at 10 and 100 μM solutions while the corresponding values for cisplatin are 85.4 and 95.3% respectively. A further study in which a medium containing 10 mM glutathione was used the complex performed even better, viz 85% inhibition of colon cancer cells at 100 μM (cisplatin 48%); 72% on breast cancer (cisplatin 10%) at 100 μM solution. 

1. A method for the preparation of a platinum(II) complex containing a neutral bidentate ligand, the method comprising the step of reacting a bis-dicarboxylatoplatinate(II) species with a neutral bidentate ligand to form a neutral dicarboxylatoplatinum(II) complex product containing a neutral bidentate ligand, wherein the neutral bidentate ligand is a diamine, a dithioether, a diseleno ether, or a heterocyclic amine with an S donor atom.
 2. The method according to claim 1, wherein the bis-dicarboxylatoplatinate(II) species and ligand are reacted at a temperature of 40° C. to 100° C. for a period of 0.5 to 3 hours.
 3. The method according to claim 2, wherein the bis-dicarboxylatoplatinate(II) species and ligand are reacted at a temperature of approximately 95° C.
 4. The method according to claim 2, wherein the bis-dicarboxylatoplatinate(II) species and ligand are reacted for approximately 1 hour.
 5. The method according to claim 1, wherein any dicarboxylatoplatinate(II) species contaminating the product is removed from the product by dissolving the product in distilled water and adding an oxalate which transforms the dicarboxylatoplatinate(II) species into a species that can be separated from the dissolved product by filtration.
 6. The method according to claim 5, wherein the oxalate is Cs₂C₂O₄.
 7. The method according to claim 1, wherein the neutral bidentate ligand is a diamine.
 8. The method according to claim 7, for the preparation of chemically and optically pure oxaliplatin, wherein the ligand is optically pure trans-1-1,2-diaminocyclohexane.
 9. The method according to claim 1, wherein the neutral bidentate ligand is a neutral bidentate heterocyclic amine with an S donor atom.
 10. The method according to claim 9, wherein the neutral bidentate heterocyclic amine includes thioethereal S.
 11. The method according to claim 10, wherein the neutral bidentate ligand is a 1-alkyl/aryl-2-alkylthioalkyl/aryl heterocyclic amine.
 12. The method according to claim 11, wherein the heterocyclic amine is an imidazole or pyridine.
 13. The method according to claim 12, wherein the neutral bidentate ligand is: Ligand (i) 1-methy1-2-methylthioethylimidazole, Ligand (ii) 1-methyl-2-methylthiopropylimidazole, Ligand (iii) 1-butyl-2-methylthiomethylimidazole, Ligand (iv) 1-methyl-2-methylthiomethylimidazole, Ligand (v) 1-butyl-2-methylthioethylimidazole, Ligand (vi) 2-methylthiomethylpyridine, Ligand (vii) 2-methylthioethylpyridine, or Ligand (viii) 2-methylthiopropylpyridine.


14. The method according to claim 1, wherein the neutral bidentate ligand is a dithioether.
 15. The method according to claim 14, wherein the neutral bidentate ligand is Ligand (xi) 2,5-dithiahexane.
 16. The method according to claim 1, wherein the neutral bidentate ligand is a diseleno ether.
 17. The method according to claim 16, wherein the neutral bidentate ligand is Ligand (xii) 2,5-diseleno hexane.
 18. An oxalatoplatinum(II) complex containing a neutral bidentate ligand having a heterocyclic amine with a thioethereal S donor atom.
 19. Oxalato(1-methyl-2-methylthioethylimidazole)platinum(II).
 20. Oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II).
 21. Oxalato(1-butyl-2-methylthiomethylimidazole)platinum(II).
 22. Oxalato(1-methyl-2-methylthiomethylimidazole)platinum(II).
 23. Oxalato(1-butyl-2-methylthioethylimidazole)platinum(II).
 24. Oxalato(2-methylthiomethylpyridine)platinum(II).
 25. Oxalato(1-amino-2-thioethylpyridine)platinum (II).
 26. Oxalato(1-amino-2-thiopropylpyridine)platinum (II).
 27. Oxalato(1-amino-2-thiomethylethane)platinum(II).
 28. Oxalato(1-amino-2-thioethylethane)platinum(II).
 29. Oxalato(2,5-dithiahexane)platinum(II).
 30. Oxalato(2,5-diseleno hexane)platinum(II).
 31. A method of treating cancer in a patient, the method comprising administering a therapeutically effective amount of the oxalatoplatinum(II) complex of claim 18 to a patient in need thereof.
 32. An oxalatoplatinum(II) complex product selected from the group consisting of oxalato(1-methyl-2-methylthioethylimidazole)platinum(II), oxalato(1-methyl-2-methylthiopropylimidazole)platinum(II), oxalato(1-butyl-2-methylthiomethylimidazole)platinum(II), oxalato(1-methyl-2-methylthiomethylimidazole)platinum(II), oxalato(1-butyl-2-methylthioethylimidazole)platinum(II), oxalato(2-methylthiomethylpyridine)platinum(II), oxalato(1-amino-2-thioethylpyridine)platinum (II), oxalato(1-amino-2-thiopropylpyridine)platinum (II), oxalato(1-amino-2-thiomethylethane)platinum(II), oxalato(1-amino-2-thioethylethane)platinum(II), oxalato(2,5-dithiahexane)platinum(II), oxalato(2,5-diseleno hexane)platinum(II), and oxaliplatin, wherein the product contains no traces of silver.
 33. A method for producing a bis-oxalatoplatinate(II) species, the method comprising the step of either reacting a platinum(II) compound or reacting a platinum(IV) compound with an oxalate at a high mole ratio of greater than 1:4.
 34. The method according to claim 33, wherein the platinum(II) or platinum(IV) compound and oxalate salt are reacted at a high mole ratio of 1:8 or greater.
 35. The method according to claim 32, wherein the platinum(II) or platinum(IV) compound and oxalate are reacted at a high mole ratio of 1:16 or greater.
 36. The method according to claim 35, wherein the platinum(II) or platinum(IV) compound and oxalate are reacted at a high mole ratio of 1:24 or greater.
 37. The method according to claim 33, wherein the platinum(II) compound is K₂PtX₄ where X is a halide.
 38. The method according to claim 33, wherein the platinum(IV) compound is K₂PtX₆ where X is a halide.
 39. The method according to claim 37, wherein X is Cl.
 40. The method according to claim 38, wherein the platinum(IV) compound is reduced to platinum(II) by the oxalate.
 41. The method according to claim 38, wherein the platinum(IV) compound is reduced by SO₂ or sulfite.
 42. The method according to claim 33, wherein the oxalate is K₂C₂O₄.
 43. The method according to claim 33, wherein the bis-oxalatoplatinate(II) species is K₂Pt(C₂O₄)₂.2H₂O.
 44. The method according to claim 33, wherein the platinum(II) compound or platinum(IV) compound and oxalate are reacted at a temperature of from 40° C. to less than 100° C. for a period of 0.5 to 4 hours.
 45. The method according to claim 44, wherein the platinum(II) compound or platinum(IV) compound and oxalate are reacted at a temperature of approximately 95° C.
 46. The method according to claim 44, wherein the platinum(II) compound or platinum(IV) compound are reacted for approximately 1 hour.
 47. The method according to claim 38, wherein X is Cl.
 48. The method according to claim 1, further comprising the step of recrystallizing the neutral dicarboxylatoplatinum(II) complex product to form a pure dicarboxylatoplatinum(II) complex containing a neutral bidentate ligand.
 49. The method according to claim 48, wherein the bis-dicarboxylatoplatinate(II) species and ligand are reacted at a temperature of 40° C. to 100° C. for a period of 0.5 to 3 hours.
 50. The method according to claim 48, wherein dicarboxylatoplatinate(II) species contaminating the product are removed from the product by dissolving the product in distilled water and adding an oxalate which transforms the dicarboxylatoplatinate(II) species into a species that can be separated from the dissolved product by filtration.
 51. The method according to claim 48, wherein the neutral bidentate ligand is an amine.
 52. The method according to claim 48, wherein the neutral bidentate ligand contains donor atoms other than N, or N together with a donor atom other than N.
 53. An oxaliplatin product containing no traces of silver. 